Field of the Invention
This invention is in the field of replacement heart valves.
Anatomy of Normal Heart Valves
There are four valves in the heart that serve to direct the flow of blood through the two sides of the heart in a forward direction. On the left (systemic) side of the heart are: 1) the mitral valve, located between the left atrium and the left ventricle, and 2) the aortic valve, located between the left ventricle and the aorta. These two valves direct oxygenated blood coming from the lungs, through the left side of the heart, into the aorta for distribution to the body. On the right (pulmonary) side of the heart are: 1) the tricuspid valve, located between the right atrium and the right ventricle, and 2) the pulmonary valve, located between the right ventricle and the pulmonary artery. These two valves direct de-oxygenated blood coming from the body, through the right side of the heart, into the pulmonary artery for distribution to the lungs, where it again becomes re-oxygenated to begin the circuit anew.
All four of these heart valves are passive structures that they do not themselves expend any energy and do not perform any active contractile function. They consist of moveable xe2x80x9cleafletsxe2x80x9d that are designed simply to open and close in response to differential pressures on either side of the valve. The mitral and tricuspid valves are referred to as xe2x80x9catrioventricular valvesxe2x80x9d because of their being situated between an atrium and ventricle on each side of the heart. The mitral valve has two leaflets and the tricuspid valve has three. The aortic and pulmonary valves are referred to as xe2x80x9csemilunar valvesxe2x80x9d because of the unique appearance of their leaflets, which are more aptly termed xe2x80x9ccuspsxe2x80x9d and are shaped somewhat like a half-moon. The aortic and pulmonary valves each have three cusps.
Since the physiological structures of native mitral and tricuspid valves and native aortic and pulmonary valves are important to this invention, they are depicted in FIG. 1, which contains a cross-sectional cutaway depiction of a normal human heart 100 (shown next to heart 100 is a segment of tubular tissue 200 which will be used to replace the mitral valve, as described below). The left side of heart 100 contains left atrium 110, left ventricular chamber 112 positioned between left ventricular wall 114 and septum 116, aortic valve 118, and mitral valve assembly 120. The components of the mitral valve assembly 120 include the mitral valve annulus 121, which will remain as a roughly circular open ring after the leaflets of a diseased or damaged valve have been removed; anterior leaflet 122 (sometimes called the aortic leaflet, since it is adjacent to the aortic region); posterior leaflet 124; two papillary muscles 126 and 128 which are attached at their bases to the interior surface of the left ventricular wall 114; and multiple chordae tendineae 13 which couple the mitral valve leaflets 122 and 124 to the papillary muscles 126 and 128. There is no one-to-one chordal connection between the leaflets and the papillary muscles; instead, numerous chordae are present, and chordae from each papillary muscle 126 and 128 attach to both of the valve leaflets 122 and 124.
The other side of the heart contains the right atrium 150, a right ventricular chamber 152 bounded by right ventricular wall 154 and septum 116, and a tricuspid valve assembly 160. The tricuspid valve assembly 160 comprises a valve annulus 162, three leaflets 164, papillary muscles 170 attached to the interior surface of the right ventricular wall 154, and multiple chordae tendineae 180 which couple the tricuspid valve leaflets 164 to the papillary muscles 170-174.
As mentioned above, the mitral valve leaflets 122 and 124, and tricuspid valve leaflets 164 are all passive structures; they do not themselves expend any energy and do not perform any active contractile function. They are designed to simply open and close in response to differential pressures on either side of the leaflet tissue. When the left ventricular wall 114 relaxes so that the ventricular chamber 112 enlarges and draws in blood, the mitral valve 120 opens (i.e., the leaflets 122 and 124 separate). Oxygenated blood flows in a downward direction through the valve 120, to fill the expanding ventricular cavity. Once the left ventricular cavity has filled, the left ventricle contracts, causing a rapid rise in the left ventricular cavitary pressure. This causes the mitral valve 120 to close (i.e., the leaflets 122 and 124 re-approximate) while the aortic valve 118 opens, allowing the oxygenated blood to be ejected from the left ventricle into the aorta. The chordae tendineae 132 of the mitral valve prevent the mitral leaflets 122 and 124 from prolapsing back into the left atrium 110 when the left ventricular chamber 114 contracts.
The three leaflets, chordae tendineae, and papillary muscles of the tricuspid valve function in a similar manner, in response to the filling of the right ventricle and its subsequent contraction.
The cusps of the aortic valve also respond passively to pressure differentials between the left ventricle and the aorta. When the left ventricle contracts, the aortic valve cusps open to allow the flow of oxygenated blood from the left ventricle into the aorta. When the left ventricle relaxes, the aortic valve cusps reapproximate to prevent the blood which has entered the aorta from leaking (regurgitating) back into the left ventricle. The pulmonary valve cusps respond passively in the same manner in response to relaxation and contraction of the right ventricle in moving de-oxygenated blood into the pulmonary artery and thence to the lungs for re-oxygenation. Neither of these semilunar valves has associated chordae tendineae or papillary muscles.
In summary, with relaxation and expansion of the ventricles (diastole), the mitral and tricuspid valves open, while the aortic and pulmonary valves close. When the ventricles contract (systole), the mitral and tricuspid valve close and the aortic and pulmonary valves open. In this manner, blood is propelled through both sides of the heart.
The anatomy of the heart and the structure and terminology of heart valves are described and illustrated in detail in numerous reference works on anatomy and cardiac surgery, including standard texts such as Surgery of the Chest (Sabiston and Spencer, eds., Saunders Publ., Philadelphia) and Cardiac Surgery by Kirklin and Barrett-Boyes.
Pathology and Abnormalities of Heart Valves
Heart valves may exhibit abnormal anatomy and function as a result of congenital or acquired valve disease. Congenital valve abnormalities may be so severe that emergency surgery is required within the first few hours of life, or they may be well-tolerated for many years only to develop a life-threatening problem in an elderly patient. Acquired valve disease may result from causes such as rheumatic fever, degenerative disorders of the valve tissue, bacterial or fungal infections, and trauma.
Since heart valves are passive structures that simply open and close in response to differential pressures on either side of the particular valve, the problems that can develop with valves can be classified into two categories: 1) stenosis, in which a valve does not open properly, or 2) insufficiency (also called regurgitation), in which a valve does not close properly. Stenosis and insufficiency may occur concomitantly in the same valve or in different valves. Both of these abnormalities increase the workload placed on the heart, and the severity of this increased stress on the heart and the patient, and the heart""s ability to adapt to it, determine whether the abnormal valve will have to be surgically replaced (or, in some cases, repaired) or not.
In addition to stenosis and insufficiency of heart valves, surgery may also be required for certain types of bacterial or fungal infections in which the valve may continue to function normally, but nevertheless harbors an overgrowth of bacteria (a so-called xe2x80x9cvegetationxe2x80x9d) on the leaflets of the valve that may flake off (xe2x80x9cembolizexe2x80x9d) and lodge downstream in a vital artery. If such vegetations are on the valves of the left side (i.e., the systemic circulation side) of the heart, embolization results in sudden loss of the blood supply to the affected body organ and immediate malfunction of that organ. The organ most commonly affected by such embolization is the brain, in which case the patient suffers a stroke. Thus, surgical replacement of either the mitral or aortic valve (left-sided heart valves) may be necessary for this problem even though neither stenosis nor insufficiency of either valve is present. Likewise, bacterial or fungal vegetations on the tricuspid valve may embolize to the lungs (resulting in a lung abscess) and therefore, may require replacement of the tricuspid valve even though no tricuspid valve stenosis or insufficiency is present. With the exception of congenital pulmonary valve stenosis or insufficiency, it is unusual for a patient to develop an abnormality of the pulmonary valve that is significant enough to require surgical repair or replacement.
Currently, surgical repair of mitral and tricuspid valves is preferred over total valve replacement when possible, although often the valves are too diseased to repair and must be replaced. Most abnormalities of the aortic valve require replacement, although some efforts are now being made to repair insufficient aortic valves in selected patients. Valve repair and valve replacement surgery is described and illustrated in numerous books and articles, including the texts cited herein.
Current Options for Heart Valve Replacement
If a heart valve must be replaced, there are currently several options available, and the choice of a particular type of prosthesis (i.e., artificial valve) depends on factors such as the location of the valve, the age and other specifics of the patient, and the surgeon""s experiences and preferences. Available prostheses include three categories of valves or materials: mechanical valves, tissue valves, and aortic homograft valves. These are briefly discussed below; they are illustrated and described in detail in texts such as Replacement Cardiac Valves, edited by E. Bodnar and R. Frater (Pergamon Press, New York, 1991).
Artificial Mechanical Valves
Mechanical valves include caged-ball valves (such as Starr-Edwards valves), bi-leaflet valves (such as St. Jude valves), and tilting disk valves (such as Medtronic-Hall or omniscience valves). Caged ball valves usually are made with a ball made of a silicone rubber (Silastic(trademark)) inside a titanium cage, while bi-leaflet and tilting disk valves are made of various combinations of pyrolytic carbon and titanium. All of these valves are attached to a cloth (usually Dacron(trademark)) sewing ring so that the valve prosthesis can be sutured to the patient""s native tissue to hold the artificial valve in place postoperatively. All of these mechanical valves can be used to replace any of the heart""s four valves. No other mechanical valves are currently approved for use by the FDA in the U.S.A.
The main advantage of mechanical valves is their long-term durability. Their main disadvantage is that they require the patient to take systemic anticoagulation drugs for the rest of his or her life, because of the propensity of mechanical valves to cause blood clots to form on them. If such blood clots form on the valve, they may preclude the valve from opening or closing correctly or, more importantly, the blood clots may disengage from the valve and embolize to the brain, causing a stroke. The anticoagulant drugs that are necessary to prevent this are expensive and potentially dangerous in that they may cause abnormal bleeding which, in itself, can cause a stroke if the bleeding occurs within the brain.
In addition to the mechanical valves available for implantation today, a number of other valve designs are described and illustrated in a chapter called xe2x80x9cExtinct Cardiac Valve Prostheses,xe2x80x9d at pages 307-332 of Replacement Cardiac Valves (Bodnar and Frater, cited above). Two of the xe2x80x9cextinctxe2x80x9d valves which deserve attention as prior art in the subject invention are the McGoon valve (pp. 319-320) and the Roe-Moore valve (pp. 320-321). Both of these involve flexible leaflets made of an elastomer or cloth coated with polytetrafluoroethylene (PTFE, widely sold under the trademark TEFLON), mounted inside a cylindrical stent. Although both were tested in humans, they were never commercialized and apparently are not being actively studied or developed today.
Artificial Tissue Valves
Most tissue valves are constructed by sewing the leaflets of pig aortic valves to a stent (to hold the leaflets in proper position), or by constructing valve leaflets from the pericardial sac (which surrounds the heart) of cows or pigs and sewing them to a stent. The stents may be rigid or slightly flexible and are covered with cloth (usually a synthetic material sold under the trademark Dacron(trademark)) and attached to a sewing ring for fixation to the patient""s native tissue. The porcine or bovine tissue is chemically treated to alleviate any antigenicity (i.e., to reduce the risk that the patient""s body will reject the foreign tissue). These tri-leaflet valves may be used to replace any of the heart""s four valves. The only tissue valves currently approved by the FDA for implantation in the U.S.A. are the Carpentier-Edwards Porcine Valve, the Hancock Porcine Valve, and the Carpentier-Edwards Pericardial Valve.
The main advantage of tissue valves is that they do not cause blood clots to form as readily as do the mechanical valves, and therefore, they do not absolutely require systemic anticoagulation. Nevertheless, many surgeons do anticoagulate patients who have any type of artificial mitral valve, including tissue valves. The major disadvantage of tissue valves is that they lack the long-term durability of mechanical valves. Tissue valves have a significant failure rate, usually appearing at approximately 8 years following implantation, although preliminary results with the new commercial pericardial valves suggest that they may last longer. One cause of these failures is believed to be the chemical treatment of the animal tissue that prevents it from being antigenic to the patient. In addition, the presence of the stent and sewing ring prevents the artificial tissue valve from being anatomically accurate in comparison to a normal heart valve, even in the aortic valve position.
Homograft Valves
Homograft valves are harvested from human cadavers. They are most commonly aortic valves but also occasionally include pulmonic valves. These valves are specially prepared and frozen in liquid nitrogen, where they are stored for later use in adults for aortic valve replacement, or in children for pulmonary valve replacement. A variant occasionally employed for aortic valve replacement is to use the patient""s own pulmonary valve (an autograft) to replace a diseased aortic valve, combined with insertion of an aortic (or pulmonary) homograft from a cadaver to replace the excised pulmonary valve (this is commonly called a xe2x80x9cRoss procedurexe2x80x9d).
The advantage of aortic homograft valves is that they, appear to be as durable as mechanical valves and yet they do not promote blood clot formation and therefore, do not require anticoagulation. The main disadvantage of these valves is that they are not available in sufficient numbers to satisfy the needs of patients who need new aortic or pulmonary valves. They also cannot be used to replace either the mitral valve or tricuspid valve. In addition, they are extremely expensive and much more difficult to implant than either mechanical or tissue valves. The difficulty in implantation means that the operative risk with a homograft valve is greater in a given patient than it is with either a mechanical or tissue valve. An additional problem is that in June 1992, the FDA reclassified homograft valves as an experimental device, so they are no longer available on a routine basis.
Principles of Artificial Heart Valve Construction
All artificial heart valves are designed to optimize three major physiologic characteristics and one practical consideration. The three major physiologic characteristics are (1) hemodynamic performance, (2) thrombogenicity, and (3) durability. The practical consideration involves ease of surgical implantation.
Multiple factors impact on each of these potential problems in the development of artificial valves. As a result, the advantage of artificial valve A over artificial valve B in one area is typically counterbalanced by valve B""s advantage in another area. If one artificial heart valve were clearly superior in all aspects to all other artificial valves in all four of these areas, it would be the only artificial valve used.
Artificial Mechanical Valves
The hemodynamic performance of mechanical heart valves has been satisfactory but not optimal, especially in the smaller sizes. All previously constructed mechanical heart valves have had some type of obstructing structure within the flow orifice of the valve when the valve is in the open position. For example, bi-leaflet valves, such as the St. Jude valve, have two bars across the orifice and in addition, the leaflets themselves are within the orifice when the valve is in the open position. Single-leaflet disc valves, such as the Medtronic-Hall valve, have a central bar and strut mechanism that keep the leaflet in place. The Bjork-Shiley valves have either one or two struts that span the valve orifice in addition to the partially-opened disc itself. The omniscience valve has the partially opened disk itself in the valve orifice when open, and the Starr-Edwards caged-ball valve has both the ball and the cage within the flow orifice of the valve in the open position. All of these structures decrease the hemodynamic performance of the mechanical valves.
Such obstructions also interfere with the normal flow patterns within and around the mechanical valve and therefore, promote thrombosis. More importantly, all artificial surfaces are thrombogenic (clot-promoting) to a greater or lesser degree. The only completely non-thrombogenic (non-clot-promoting) surface that exists is the layer of viable endothelial cells that line the interior of all the body""s vascular surfaces, including the inside of the heart chambers and the native valve leaflets. Therefore, any metal or plastic material, no matter how highly polished, will have some level of thrombogenicity unless the surface of the artificial material can be covered with endothelial cells. It is for this reason that all patients with artificial mechanical heart valves must be permanently anticoagulated.
The major advantage of mechanical valves over tissue valves is long-term durability. Mechanical valve construction has been based on sophisticated engineering principles that have proven to be sound in terms of providing devices that are extremely resistant to wear and structural failure. Nevertheless, structural failure of mechanical valves does occur and it is the major reason for the recent withdrawal from the market of two commercially available mechanical valves (the Bjork-Shiley Concavo-Convex(trademark) single disc valve and the Duramedics(trademark) bi-leaflet valve).
Artificial Tissue Valves
Under the best of circumstances (i.e., replacement of the aortic valve), the construction of artificial tissue valves has been based on the concept that if the artificial valve can be made to approximate the anatomy (form) of the native valve, then the physiology (function) of the artificial valve will also approximate that of the native valve. This is the concept that xe2x80x9cFunction Follows Form.xe2x80x9d For example, the manufacturers of all artificial porcine valves first re-create the form of a native human aortic valve by: 1) harvesting a porcine aortic valve, 2) fixing it in glutaraldehyde to eliminate antigenicity, and 3) suturing the porcine valve to a stent to hold the three leaflets in place. In other words, the primary goal in the construction of these artificial valves is to reproduce the form of the human aortic valve as closely as possible. The assumption is made that if the artificial valve can be made to look like the human aortic valve, it will function like the human aortic valve (i.e., proper function will follow proper form). The same assumption is also followed for commercially available pericardial valves.
In the case of mitral or tricuspid valve replacement, even the dubious concept of xe2x80x9cfunction follows formxe2x80x9d has been discarded since the same artificial valves that are designed to look like the aortic valve are used to replace the mitral and tricuspid-valves. In other words, no attempt at all is made to reproduce even the form of these native valves, much less so their function.
Thus, in the case of artificial valves to be used for aortic valve replacement, the dubious concept of xe2x80x9cfunction follows formxe2x80x9d has dictated the construction of all artificial tissue valves during the 30 years of their development and use. Even worse, no discernable underlying concept at all has been used in terms of the artificial valves used to replace the mitral and tricuspid valves.
The xe2x80x9cFunction Follows Formxe2x80x9d concept has several limitations and appears to be a fundamental shortcoming which underlies the present construction of all artificial tissue valves. In the first place, it simply is not possible to recreate the exact anatomy (form) of a native heart valve utilizing present techniques. Although homograft (human cadaver) and porcine aortic valves have the gross appearance of native aortic valves, the fixation process (freezing with liquid nitrogen, and chemical treatment, respectively) alters the histologic (microscopic) characteristics of the valve tissue. Porcine and bovine pericardial valves not only require chemical preparation (usually involving fixation with glutaraldehyde), but the leaflets must be sutured to cloth-covered stents in order to hold the leaflets in position for proper opening and closing of the valve. A recent advance has been made in this regard by using xe2x80x9cstentlessxe2x80x9d porcine valves that are sutured directly to the patient""s native tissues for aortic valve replacement, but the problem of chemical fixation remains. In addition, these stentless artificial valves cannot be used for mitral or tricuspid valve replacement.
Perhaps the major limitation of the xe2x80x9cFunction Follows Formxe2x80x9d concept is that no efforts have been made previously to approximate the form of either the mitral valve or the tricuspid valve. If animal tissue valves are used to replace either of these native valves, the tri-leaflet porcine aortic valve prosthesis or the tri-leaflet bovine pericardial valve prosthesis is normally used. In doing so, even the faulty concept of xe2x80x9cFunction Follows Formxe2x80x9d is ignored, since there are no artificial valves available for human use that approximate the anatomy (form) of the native mitral or tricuspid valves.
The nearest attempt at reproducing the function of the native mitral valve was reported by Mickleborough et al in 1989. These tests involved the use of commercially-prepared sheets of pericardial tissue from cows, which had been treated with glutaraldehyde before storage and shipping. A longitudinal suture line was used to convert the flat sheet of tissue into a cylinder, then two triangular regions were removed from one end of the cylinder, to generate two flaps. The inlet end was sutured to the mitral valve annulus, while the two tissue flaps at the carved outlet end were sutured to the papillary muscles.
The mitral valve disclosed by Mickleborough et al suffers from a drawback which is believed to be important and perhaps even crucial to proper valve functioning. In a properly functioning natural valve, the anterior leaflet does not have its center portion directly attached to the anterior papillary muscle via chordae. Instead, the anterior leaflet is attached to both the anterior and posterior papillary muscles, via chordae that are predominantly attached to the peripheral edges of the leaflet. In the same manner, a native posterior leaflet is attached to both the anterior and posterior papillary muscles, via chordae that are predominantly attached to the peripheral edges of the leaflet. As a result, the line of commissure (closure) between the two mitral leaflets when the valve is closed during systole is oriented in roughly the same direction as an imaginary line that crosses the tips of both papillary muscles. This orientation of the leaflets and papillary muscles is shown in illustrations such as page 11 of Netter 1969. This natural orientation can be achieved in the valve of the subject invention as depicted in FIGS. 2 and 3, discussed below.
By contrast, the replacement valve described by Mickleborough et al alters and distorts the proper orientation of the replacement leaflets. Mickleborough""s approach requires each sculpted leaflet to be trimmed in away that forms an extended flap, which becomes a relatively narrow strand of tissue near its tip. The tip of each pericardial tissue strand is sutured directly to a papillary muscle, causing the strand to mimic a chordae tendineae. Each strand extends from the center of a leaflet in the Mickleborough et al valve, and each strand is sutured directly to either an anterior and posterior papillary muscle. This requires each leaflet to be positioned directly over a papillary muscle. This effectively rotates the leaflets of the Mickleborough valve about 90xc2x0 compared to the leaflets of a native valve. The line of commissure between the leaflets, when they are pressed together during systole, will bisect (at a perpendicular angle) an imaginary line that crosses the peaks of the two papillary muscles, instead of lying roughly along that line as occurs in a native valve.
There has been no indication since the publication of Mickleborough et al 1989 that their approach is still being studied (either by them, or by any other research team), and there has been no other indication during the intervening years that their approach is likely to lead to a valve replacement technique for actual use in humans.
It should be noted that one of the primary goals of Mickleborough and her associates apparently was to propose a new way to maintain continuity between the valve annulus and the papillary muscles. It was first proposed about 30 years ago (by C. W. Lillehei and perhaps by others as well) that proper muscle tone of the left ventricular wall, and proper postoperative ventricular functioning, required a tension-bearing connection between the mitral valve annulus and the papillary muscles on the inside of the ventricular wall. This suggestion was widely ignored in the design of replacement mitral valves, which required excision of the chordae tendineae without making any effort to provide a substitute that would keep the ventricular wall coupled to the valve annulus. However, various studies (such as Rittenhouse et al 1978, David 1986, Hansen et al 1987, and Miki et al 1988) continued to indicate that the tension-conveying role of the chordae was important to proper ventricular function. Based on those studies, Mickleborough et al apparently were attempting to create and propose a new valve design that could accomplish that goal. They did indeed accomplish that goal, and the apparent lack of any follow-up or commercialization of their design presumably was due to other problems, such as the altered orientation of the leaflets in their design.
A different approach to creating artificial tissue valves is described in articles such as Love and Love 1991, and in U.S. Pat. No. 5,163,955 (Calvin et al 1992) and U.S. Pat. No. 4,470,157 (Love 1984). In that research, surgeons harvested a piece of pericardial tissue from the same animal that was to receive the artificial valve. Such tissue, if harvested from the same human body that will receive the implant, is referred to as autologous or autogenous (the terms are used interchangeably, by different researchers). Using a cutting die, the pericardial tissue was cut into a carefully defined geometric shape, treated with glutaraldehyde, then clamped in a sandwich-fashion between two stent components. This created a tri-leaflet valve that again resembles an aortic or pulmonary valve, having semilunar-type cusps rather than atrioventricular-type leaflets. These valves were then tested in the mitral (or occasionally tricuspid) valve position, using sheep.
Although those valves were structurally very different from the valves of the subject invention, the Love and Love article is worth attention because it discusses chemical fixation. They used glutaraldehyde treatment even though their tissue source was from the same animal and was therefore non-antigenic, because earlier reports and tests had suggested that some types of untreated autologous tissue suffer from thickening and/or shrinkage over time. Love and Love suggested that glutaraldehyde can help such tissue resist such changes, apparently by forming crosslinking bonds that tend to hold adjacent collagen fibers in a fixed-but-flexible conformation. This use of glutaraldehyde fixation as a treatment to reduce shrinkage or other physical distortion (as distinct from using it as a method of reducing tissue antigenicity) is an old and well-established technique for treating non-autologous tissue, but whether it is also beneficial for treating autologous tissue has not yet been extensively evaluated. The effects of chemical fixation of intestinal or other tubular tissue used to create heart valves as described herein can be evaluated by routine experimentation.
Another report describing the use of autologous tissue to reconstruct mitral valves is Bailey et al 1970. However, Bailey et al focused on repairing rather than replacing mitral valves, usually by cutting an incision into one or both leaflets and then inserting a segment of tissue into the incision to enlarge the leaflet(s).
Physiologic Factors and in Utero Development
The subject invention relates to a method of using tubular starting material to replace any of the four heart valves during cardiac surgery. This approach is supported by and consistent with a fundamental principle of native heart valve function, which either went unrecognized in previous efforts to develop replacement valves, or which was sacrificed and lost when compromises were required to adapt available materials to surgical requirements.
The basic principle, which deserves repeated emphasis because it has been so widely disregarded by other efforts in this field, is that Form Follows Function. In one manifestation of this principle, if an artificial valve can be created that can truly function like a native valve, its resultant form will be similar to that of the native valve.
A highly important observation by the Applicant that contributed to the recognition of the pervasive and overriding importance of this principle was the following: the entire cardiovascular system, including the heart, begins in utero as a single, relatively straight tube of tissue. Anatomical drawings depicting the in utero development of the heart are available in numerous scientific publications and books, including Netter 1969. As shown in those figures (or similar figures available in other medical reference works), the so-called xe2x80x9cheart tubexe2x80x9d is readily discernible by the 23rd day of gestation. This tube will eventually develop into the entire cardiovascular system of the body. The tissue that exists between the portion of the tube destined to become the ventricles, and the portion that will become the atria, is where the mitral and tricuspid valves will ultimately form. This region of tissue is in a tubular form.
The heart tube undergoes a process of convolution beginning at approximately 25 days gestation. This convolution of the heart tube forms what is called the xe2x80x9cheart loopxe2x80x9d and is responsible for the aortic valve ultimately coming to lie adjacent to the mitral valve. When a mature mitral valve is viewed from the atrial side, the anterior portion of the mitral valve annulus is relatively flat. This distortion of the original roundness of the mitral annulus is caused by the presence of the aorta against the anterior mitral valve. It is also the reason that the anterior leaflet of the mitral valve is contiguous with the aortic valve annulus. Finally, it explains why accessory atrioventricular connections (accessory pathways) that occur in the Wolff-Parkinson-White syndrome never occur in this portion of the mitral valve annulus; this is the only portion of the entire atrioventricular groove on either side of the heart where the atrium and ventricle were never contiguous during fetal development.
By approximately 56 days gestation, the heart tube development reaches a stage that displays a first constricted tube region between the primordial right atrium and the primordial right ventricle (this portion of the tube will become the tricuspid valve) and a second constricted tube region between the primordial left atrium and primordial left ventricle (the future mitral valve).
As the developing heart of a fetus undergoes various convolutions, septations, and compartmentalizations, the tissues that are to eventually become the heart valves maintain their tubular structure. Prior to the onset of fetal heart function, portions of the walls of these tubular structures undergo a process of dissolution, leaving behind only those portions of the original tubes that are necessary for the proper functioning of the heart. This dissolution also affects the ventricular walls as they rapidly enlarge in size; if it did not, the walls would become prohibitively thick as the physical size of the heart increased, and the heart could not function effectively as a pump since it would become simply a large mass of ventricular muscle.
The dissolution process also operates on the tubular constrictions that will become the four heart valves. In the case of the semilunar valves (the aortic and pulmonary valves), the necessary functional remnants are the three cusps, which are the remains of the functioning portion of a simple tube. This principle is strengthened by the fact that although frequent reference is made to the pulmonic or aortic valve xe2x80x9cannulusxe2x80x9d, knowledgeable anatomists are quick to point out that there is no such anatomical structure. The thickened tissue that is commonly referred to as the xe2x80x9cannulusxe2x80x9d of these valves is simply the flexion point of the three cusps, the remnants of a simple tube that is fixed at three points distally and subjected to uniform pressure on its outside, resulting in collapse of the tube on the three sides between the points of distal fixation, which in turn, results in three nearly identical cusps. All tissue other than these moveable and functional cusps has undergone the normal process of dissolution as the aorta and pulmonary artery have enlarged, leaving behind only that tissue recognized as the cusps of these semilunar valves.
At the mitral and tricuspid valves locations, the dissolution process leaves behind the valve leaflets, chordae tendineae, and papillary muscles in both the right ventricle (tricuspid valve) and left ventricle (mitral valve). In other words, that portion of the original tube that is necessary for the development of the native heart valves is spared the dissolution process and the rest of the tube dissolves away. The valve leaflets are tube remnants, which are attached circumferentially to the fibrous annulus of the heart at their base and attached by chordae tendineae (additional tube remnants) at their free edges to papillary muscles (still more tube remnants) inside the ventricles. The leaflets, chordae tendineae, and papillary muscles of each the two A-V valves represent the necessary functional remnants of the original in utero tubular structures of the heart.
Using xe2x80x9cForm Follows Functionxe2x80x9d as a basic guiding principle, the present invention is based on the realization that a tubular structure having proper size and suitable material characteristics, if placed inside a mitral or tricuspid valve annulus after excision of the native valve (or inside an aorta or pulmonary artery, as described below) will function exactly like the normal valve in that position, assuming proper fixation of the inlet and outlet ends of the tube. The xe2x80x9cForm Follows Functionxe2x80x9d principle predicts that if the intended function of a replacement valve is to emulate the performance and function of a native mitral or tricuspid valve, then the form of a replacement valvexe2x80x94the structure and appearance of the replacement valvexe2x80x94should resemble the form of a native mitral or tricuspid valve. Since the native valves are generated from tubular starting material during fetal development, this principle further suggests that replacement valves should also be generated from tubular material.
This principle is given added support by the results that were observed in an artificial tissue valve that had been implanted into the mitral valve position in a human heart. The Applicant learned of these results during a presentation by Professor Donald Ross of the National Heart Hospital and Brompton Hospital (London, England), the cardiac surgeon who had performed that surgery. The implanted valve was originally a commercially available trileaflet tissue valve that was implanted into the mitral position in a 35-year-old female. The trileaflet valve had been constructed using fascia late tissue (a relatively tough and flexible layer of tissue that normally surrounds certain types of muscles) which had been sewn into a circular stent. After 5 years, the artificial valve had to be removed because its leaflets had become calcified and immobile, resulting in both mitral stenosis and mitral insufficiency. Upon exposing the artificial valve during the removal surgery, the surgeon was struck by the similarity in shape and appearance of the diseased trileaflet valve to a normal mitral valve. The commissures of the three leaflet artificial tissue valve had fused in a manner so that two leaflets had been formed: one large anterior leaflet, and one smaller posterior leaflet, as seen in a native mitral valve. Furthermore, the commissure between the two leaflets when the patient""s valve was closed by back pressure closely resembled the semi-circular commissure formed by leaflets in a native mitral valve.
During the presentation by Professor Ross, the Applicant witnessed a picture showing how the three-leaflet artificial valve had been converted into a bi-leaflet valve during the course of five years inside a human heart. It became clear to the Applicant that the patient""s heart had been attempting to make the valve conform to the heart""s functional needs.
Prior to witnessing that presentation, the Applicant had already been considering the question of whether tubular tissue might be useful for creating replacement heart valves. After seeing Prof. Ross""s photographs, which provided strong physiological confirmation of the xe2x80x9cForm Follows Functionxe2x80x9d principle, the Applicant began to carry out experiments to assess the possibility of using tubular tissue to replace heart valves. In a simple mechanical test, he obtained some highly flexible rubber tubes by cutting off the fingers of surgical gloves, then he sculpted the finger tubes to resemble the leaflets of mitral or tricuspid valves, then he sutured the sculpted rubber tubes inside of slightly larger tubes made of Dacron(trademark). An internal rubber tube was secured proximally around the entire periphery of a tube, to emulate a valve annulus, and the sculpted rubber flaps at the distal ends were coupled to the tube walls by means of loose suture strands that emulated chordae tendineae. When cyclical pressure was generated by attempting to blow and then suck air through the tube, the interior rubber leaflets opened and closed in a manner that looked identical to natural mitral or tricuspid leaflets opening and closing. This provided additional confirmation of the xe2x80x9cForm Follows Functionxe2x80x9d principle.
The physiologic principle that the functional components of native heart valves are the remnants of simple tissue tubes, and the idea of using tubular structures to replace defective heart valves, has been completely ignored in the design and construction of all replacement valves in use today. Indeed, although xe2x80x9cForm Follows Functionxe2x80x9d is a well-respected principle in fields such as engineering or evolutionary studies, it is often disregarded among medical researchers, some of whom apparently seem to feel that efforts to sever or reverse this relationship represent triumphs of technology over nature. As an example, kidney dialysis machines, which look nothing like normal kidneys, are a purely technological, non-natural solution; they use a completely artificial form to generate and provide a certain needed function. However, as any dialysis patient would attest, they fall far short of being truly optimal.
In a similar manner, all artificial heart valves in use today, whether tissue or mechanical, have been designed based on the belief that either: 1) function can be forced to follow form (aortic and pulmonary valve replacement), or 2) neither function nor form of the native valve can be reproduced, so a replacement valve (either tissue or mechanical) must merely function as a one-way passive valve (mitral and tricuspid valve replacement). In the case of artificial tissue valves, the form of an artificial valve is established first, in the hope that the valve will function in a manner similar to a native valve. In the case of artificial mechanical valves, the disruption of the interaction between form and function goes even farther, and the caged balls, hinged flappers, and other devices in mechanical valves have even less physical similarity to native valves. However, the problems in both of these approaches are evident in the limitations suffered by every type of replacement valve that is in use today.
There is another way to express the concept of xe2x80x9cForm Follows Functionxe2x80x9d which may help explain it to people who would point to mechanical heart valves, dialysis machines, and other non-natural forms that have been used to mimic the function of body parts. In such examples, function is forced to follow form. In crude and simple terms, the function of a heart valve is merely to allow flow in one direction only. Any type of mechanical check valve with a caged-ball or flapper-and-seat design can provide that level of function.
However, when the long-term aspects of heart valve function are also taken into account (including the functions of providing low hemolysis, low turbulence, avoiding calcification, etc.), it becomes clear that artificial forms cannot fully provide those functions. The best and perhaps only way to provide a replacement valve with the complete, long-term functionality of a natural heart valve is by giving proper deference to the relationship between function and form.
This principle can be stated as, xe2x80x9cForm and function form a cycle.xe2x80x9d Each follows the other, but each also precedes and affects the other. If either half of this cycle is violated or disrupted, it will create problems that will stand in the way of an optimally functional, reliable, durable system with minimal hemolysis, turbulence, and calcification. On a short-term basis, function can be forced to adapt to an unnatural form; however, any such short-term solution will be plagued by problems and limitations over the long run. The problems and shortcomings of current mechanical replacement valves are a clear and direct demonstration of this principle.
The following series can help to illustrate the principle, xe2x80x9cForm and function form a cycle.xe2x80x9d First, a form is created: tubular tissue is used to create a new mitral valve. This form then creates a function: the new valve allows flow in only one direction, from the atrium to the ventricle. This function, in turn, creates another form: the leaflets of the new mitral valve will close in a xe2x80x9csmilexe2x80x9d configuration resembling a native mitral valve during closure. This secondary form then creates a secondary function: the new valve will provide good long-term use and low levels of turbulence, hemolysis, calcification, and leaflet stress. Form and function form a cycle, and this cycle cannot be disrupted by injecting and imposing an artificial, unnatural form in the heart without impeding the ability of proper form and proper function to interact with, support, and enhance each other.
In addition, certain items of evidence suggest that conventional replacement tissue valves, which cause high levels of turbulence, contribute to the important problem of leaflet calcification. The correlation between high turbulence and leaflet calcification is discussed below.
On the basis of the physiological facts, observations, and principles described above, and on the basis of experiments carried out by the Applicant, it appears that if heart valves are damaged or diseased to the point of requiring replacement, they should be replaced by tubular structures which function like native heart valves.
Accordingly, one object of this invention is to provide a replacement heart valve including a tubular segment of material that more closely resembles the construction and function of a native heart valve.
This and other objects and advantages of the invention will become clear as the invention and certain preferred embodiments are described below and in the drawings.
In accordance with one aspect of the present invention, a prosthetic heart valve comprises at least two cusps comprised of a thin and flexible material. Each cusp has an inner surface and an outer surface and is attached to another cusp along a longitudinal suture line. Each of the cusps is adapted to flex inwardly into and out of engagement with another cusp so as to close and open the valve in response to force by blood pressure. A portion of the inner surface of one cusp is in a facing relationship with a portion of the inner surface of another cusp adjacent the longitudinal suture line when the valve is open.
In accordance with another aspect, the present invention discloses a method for making a prosthetic heart valve. A section of substantially flat, flexible material is provided, and at least two leaflets are cut out of the flat material. Each of the leaflets has an inner face, an outer face, an in-flow end, an out-flow end, and side edges. The side edges of adjacent leaflets are sewn together so as to form a substantially tubular valve structure having an in-flow end and an out-flow end. Each leaflet is configured so that a portion of the inner face of each leaflet is in a facing relationship with a portion of the inner face of an adjacent leaflet.